Help: Air to Water Heat Exchanger Calculation

Brianm

Hello,



I have a coal stove and want to add a heat exchanger to it to supply heated

water to the house’s hydronic heating system. 

I intend to use coiled tubing for the heat exchanger and would like to

calculate roughly how many feet of tubing I will need to transfer 100,000

BTU/hour.



 I have found lots of good information online; and I have tried using some of the

equations I found and ended up with some extremely unreasonable numbers.



 What I am hoping to receive is an equation that I can then plug into an excel

worksheet that will let me try various combinations.  (My preference is for an equation that does not have calculus or logarithmic factors, and one that accepts non metric

units,  feet, inches, BTU per hour ect.)



 

My variables



 Tubing Type:   304 SS,  316L,  Copper Tubing, or Carbon Steel.  (My preference will be for 316L)





Tubing Wall Thickness: .020 to .060”

 



Tubing Outside Diameter:  .5” to .875”





Total length of tubing:    25 to 100 feet  (hopefully)





 Transfer media:   Water  (inside tubing),   Air (combustion chamber gases)



A.  Main exchanger coils would be  Air to Water



B.  And then for domestic hot water loop: Water to water



<span style="text-decoration:underline;">Temperatures of media:</span>

* Water Average =145°F  (returning water should be about 100°F, which is heated to about 190°F)



*Air in the combustion chamber = 400°F  (my best estimate, it would be much hotter directly over the fire, and cooler near the walls)



Thanks, Brian